TokuDB: memory leak in external_lock

# It's not necessary to install TokuDB at runtime, 

# instead the server can be run with --plugin-load=ha_tokudb

INSTALL SONAME 'ha_tokudb';

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a INT) ENGINE=TokuDB;

INSERT INTO t1 VALUES (1),(2);

UNINSTALL SONAME 'ha_tokudb';

# Let server to shutdown (with MTR, it happens on its own)

==2806== 212 bytes in 1 blocks are still reachable in loss record 9 of 10

==2806==    at 0x4C28CCE: malloc (vg_replace_malloc.c:292)

==2806==    by 0xE3853F: sf_malloc (safemalloc.c:115)

==2806==    by 0xE276A6: my_malloc (my_malloc.c:100)

==2806==    by 0xD63B40C: ???

==2806==    by 0xD63EEA2: ???

==2806==    by 0xD64F7DE: ???

==2806==    by 0x869CC7: handler::ha_external_lock(THD*, int) (handler.cc:5843)

==2806==    by 0x93F723: lock_external(THD*, TABLE**, unsigned int) (lock.cc:357)

==2806==    by 0x93F4A3: mysql_lock_tables(THD*, st_mysql_lock*, unsigned int) (lock.cc:309)

==2806==    by 0x93F3B4: mysql_lock_tables(THD*, TABLE**, unsigned int, unsigned int) (lock.cc:276)

==2806==    by 0x61937F: lock_tables(THD*, TABLE_LIST*, unsigned int, unsigned int) (sql_base.cc:5310)

==2806==    by 0x618B3A: open_and_lock_tables(THD*, TABLE_LIST*, bool, unsigned int, Prelocking_strategy*) (sql_base.cc:5081)

==2806==    by 0x60D1E2: open_and_lock_tables(THD*, TABLE_LIST*, bool, unsigned int) (sql_base.h:485)

==2806==    by 0x655BE1: mysql_insert(THD*, TABLE_LIST*, List<Item>&, List<List<Item> >&, List<Item>&, List<Item>&, enum_duplicates, bool) (sql_insert.cc:722)

==2806==    by 0x676262: mysql_execute_command(THD*) (sql_parse.cc:3471)

==2806==    by 0x67E48E: mysql_parse(THD*, char*, unsigned int, Parser_state*) (sql_parse.cc:6452)